1. Field of the Invention:
This invention relates generally to the injection molding art and more particularly to a new and improved method of making a mold pin insert for a high quality, such as optical quality, injection mold for molding high or optical quality parts.
2. Discussion of the Prior Art:
As will become readily evident as the description proceeds, the principles of the invention may be utilized to make pin inserts for various types of molds. The invention, however, is particularly adapted to making mold pin inserts for optical quality molds of the kind which are employed to mold intraocular lenses, contact lenses and the like. For this reason, the invention will be described in the context of such use.
A typical optical quality mold for molding optical lenses, such as intraocular lenses and contact lenses, comprises a pair of mold plates to be mounted on the confronting sides of the platten and base of an injection molding machine. Positioned within these plates, on a common axis normal to the plates, are a pair of tubular mold inserts having external shoulders which position the inserts axially within the plates with their adjacent inner ends flush with the confronting parting faces of the plates. Removably fitted within the inserts are mold insert pins and insert bases between and in seating contact with the mold plates and the outer ends of the pins. The inner ends of these pins are flush with the inner ends of the mold inserts and the parting faces of the mold plates.
The inner end faces of the mold insert pins abut one another and are shaped to define a mold cavity when the die plates are closed. For molding a plano-convex lens, the inner end face of the lower mold pin is flat and normal to the pin axis. The inner end face of the upper pin forms a concave recess. When the die plates are closed, these faces together form a plano-convex mold cavity. Obviously, the pin faces may be shaped to form other types of mold cavities. The plastic to be molded enters the cavity thru an injection sprue in the mold plates.
The present invention is concerned with improving the mold insert pins and, to this end, provides an improved method for making these pins. According to the existing pin fabricating methods of which I am aware, mold insert pins are made from rolled steel rods whose diameter is just slightly larger than the desired outside diameter of the finished pins. Each rod is then ground or otherwise machined to the finished size. The initial rod diameter is selected to be only slightly larger than the finished pin diameter for reasons of machining economy, i.e., to minimize the machining time and the material lost in the machining operation.
The machined rod is then commonly sent out to a commercial heat treatment facility without specifying any particular heat treatment program. In many cases, the commercial heat treatments involve cryogenic freezing of the workpiece in the heat treatment cycle.
The end mold surface is then formed on the heat treated pin. This surface is commonly formed utilizing an electric discharge machining operation or a lathe turning operation.
This existing mold insert pin fabrication technique has many disadvantages. Thus, as noted earlier, roll forming a steel rod, such as the rods from which the existing pins are made drives impurities in the rod toward the center of the rod. This creates within the rod a central region of high impurity concentration which is exposed at the end mold face of the finished pin. As a consequence of this and the method of polishing of the mold face after electrical discharge machining, the mold face has a relatively high concentration of pits which result in bumps or peaks and the like in the finished molded lens. These bumps or peaks, while relatively small, cause much eye irritation and create pits in the eye tissue which often result in eye infection. By way of example, the industry standard for optical quality mold insert pins is a minimum of 4 to 10 pits on each mold surface with a pit size up to 0.06 microns. By contrast, the present invention results in a maximum of 3 pits, and usually one or two pits, and a maximum pit size on the order of 0.006 microns.
The commercial heat treatment programs, while suitable for many purposes, usually result in large grain size with impurities between the grains. The cryogenic freezing steps which are often used in these programs and are accepted as routine procedures, aggravate the problem due to the molecular stress produced by such freezing.
As noted above, electric discharge machining of the pin mold face and the methods used thereafter polish the face together with the high impurity concentration region at the center of the region result in the formation of a relatively large number of relatively large pits in the face which create eye irritating and often damaging peaks in the finished molded lens. In this regard, electric discharge machining of the mold surface has been found to produce deep fracturing of the surface due to the lack of any effective heat treatment after machining. Lathe turning of the mold surface, on the other hand, is commonly performed in a way which yields a free cutting action specifically for the purpose of avoiding edge build up before the cutter which the present invention deliberately employs to reorient the grains circularly about the pin axis and smooth out the pits and pipes, as explained later. These disadvantages of the existing methods of forming the pin mold face are aggravated by the high impurity concentration region at the center of the pin exposed at the mold face since the impurities tend to be dislodged by the face forming operations, thereby creating pits in the face which must be removed by polishing.
The existing polishing procedures also create a problem, however. Thus, the customary industry procedure for polishing the pin mold face involves a lapping operation utilizing an aluminum or diamond dust paste. Because of the high concentration of impurities at the center of the face, the polishing operation often tears impurity particles from the face, thereby forming additional pits which must be removed by more polishing.
Because of these many disadvantages of the existing mold insert pin fabricating procedures, the pin discard rate is very high. At best, the procedures produce mold pins capable of producing somewhat undesirable molded lenses and the like. Accordingly, there is a definite need for an improved pin fabricating method.